Redox potential, toxicity

Cultivation of strictly anaerobic organisms requires not only that the medium be oxygen-free, but also that the redox potential of the medium be compatible with that required by the organisms. This may be accomplished by addition of reducing agents such as sulfide, dithionite, titanium(III) citrate, or titanium(IIl) nitrilotriacetate. Any of these may, however, be toxic so that only low concentrations should be employed. Attention has been drawn to the fact that titanium(III) citrate-reduced medium may be inhibitory to bacteria during initial isolation (Wachenheim and Hespell 1984). 

The limitation for chemical oxidation is that oxidation is frequently not completed to the final products C02 and H20. This can be due to a number of factors, including oxidant concentration, pH, redox potential, or the formation of stable intermediate toxic oxidation products. 

Other factors affecting performance include the presence of toxic material, the redox potential, salinity of the groundwater, light intensity, hydraulic conductivity of the soil, and osmotic potential. The rate of biological treatment is higher for more permeable soils or aquifers. Bioremediation is not applicable to soils with very low permeability, because it would take a long time for the cleanup process unless many more wells were installed, thus raising the cost. 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

Although no good quantitative correlation between redox potentials of flavonoids and their prooxidant activities still was not documented, a relationship between the prooxidant toxicity of flavonoids to HL-60 cells and redox potentials apparently takes place [176]. However, there is a simple characteristic of possible prooxidant activity of flavonoids, which increases with an increase in reactive hydroxyl groups in the B ring. From this point of view, the prooxidant activity of flavonoids should increase in the range kaempferol < quercetin < myricetin (Figure 29.7). Thus, for many flavonoids the ratio of their antioxidant and prooxidant activities must depend on the competition between Reactions (14) and (15) and Reaction (17). 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

Design concepts are now being applied more effectively to mineral supplements. For example, by controlling the redox potential of iron, toxic effects associated with excess Fe(II) during parental supplementation can be avoided. Peroxovanadate complexes can inhibit insulin-receptor-associated phosphotyrosine phosphatase and activate insulin receptor kinase, and both V(IV) and V(V) offer promise as potential insulin mimics. 

Gambrell, R.P., Taylor, B.A., Reddy, K.S., and Patrick, W.H., Jr. Fate of selected toxic compounds nnder controlled redox potential and pH conditions in soil and sediment-water systems, U.S. EPA Report 600/3-83-018, 1984. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Metronidazole, a nitroimidazole, is effective only against anaerobic bacteria since its mechanism of action involves the generation of toxic metabolites in a milieu of low redox potential. It is well absorbed when administered orally and, apart from disulfiram reactions when co-administered with alcohol, is well tolerated. It is indicated in infections in which anaerobes have a major role, such as intestinal or biliary tract sepsis, and is the first-line agent for C. of/ffic/Ze-associated colitis. 

These proteins are important for binding potentially toxic metals such as cadmium, mercury, and lead, which all bind to sulfydryl groups. Consequently, the binding and removal of these metals are protective functions. Metallothioneins are markedly induced by cadmium exposure and the small protein, rich in SH groups, can then sequester the metal. They also may have a protective role in oxidative stress and protect redox-sensitive processes. The protein also has a role in cadmium nephrotoxicity (see chap. 7). 

Methoxy-8-hydroxylaminoquinoline, an N-hydroxylated metabolite of primaquine (Fig. 7.46), is directly toxic, causing hemolysis and methemoglobinemia in rats. However, there are several pathways of metabolism for primaquine and several potential toxic metabolites. Thus, hydroxylation of primaquine at the 5-position of the quinoline ring also forms redox-active derivatives able to cause oxidative stress within normal and G6PD-deficient human red cells as well as rat erythrocytes (Fig. 7.46). 

In isomer 1, where catalytic redox functions are retained, a facilitated inactivation reaction such as oxazole formation, which can take place even nonenzymatically in any cell, results in potential toxicity. 

The herbicidal activity of the bipyridyliums depends on their redox properties. Their abilities as one-electron acceptors of the right redox potential (-350 mV for diquat and -450 mV for paraquat) allow them to siphon electrons out of the photosynthetic electron-transport system, competing with the natural acceptors. The radical anion produced is then reoxidized by oxygen, generating the real toxicant, hydrogen peroxide, which damages plant cells. Structure-activity relationships in this series have been reviewed (60MI10701). 

For many years, the cytoprotective effects of plant phenolic compounds were attributed to their ability to direct scavenge oxidants and free radicals. However, as discussed in the last section, this concept is oversimplified and misleading. More and more evidence suggess that plant phenolic compounds could interact with cellular components and trigger a series of cellular responses, which are able to modulate the redox status of the cells and protect the cells from potentially toxic electrophiles/oxidants. 

The initial species present, their relative concentrations, the induction of their enzymes, and their ability to acclimate once exposed to a chemical are likely to vary considerably, depending upon such environmental parameters as temperature, salinity, pH, oxygen concentration (aerobic or anaerobic), redox potential, concentration and nature of various substrates and nutrients, concentration of heavy metals (toxicity), and effects (synergistic and antagonistic) of associated microflora (Howard and Banerjee, 1984). Many of the parameters affect the biodegradation of chemicals in the environment as well as in biodegradation test systems used to simulate the environment. 

It is generally accepted that free ionic forms of heavy metals are generally more toxic to biota than chelated or precipitated forms. Several factors control metal bioavailability and, thus, toxicity in environmental samples. These factors include pH, redox potential, alkalinity, hardness, adsorption to suspended solids, cations and anions, as well as interaction with organic compounds (Kong et al., 1995). 

---